fixed default coroutine selection for musl/uclibc

https://github.com/ruby/ruby/pull/3567/commits/b570e7de87aaad8c903176d835e8124127f627b3

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>

diff -Nur ruby-3.0.0.orig/configure.ac ruby-3.0.0/configure.ac
--- ruby-3.0.0.orig/configure.ac	2020-12-25 04:33:01.000000000 +0100
+++ ruby-3.0.0/configure.ac	2021-02-22 13:00:53.990314464 +0100
@@ -2441,7 +2441,10 @@
             rb_cv_coroutine=copy
         ],
         [
-            rb_cv_coroutine=ucontext
+        AC_CHECK_FUNCS([getcontext swapcontext makecontext],
+               [rb_cv_coroutine=ucontext],
+               [rb_cv_coroutine=copy; break]
+	       )
         ]
     )
     AC_MSG_RESULT(${rb_cv_coroutine})
